Genotype–environment correlations: Implications for determining the relationship between environmental exposures and psychiatric illness
By: Jaffee, S. R., & Price, T. S. (2008)
Psychiatry, 7, 496–499

Abstract

Psychological risk factors for psychiatric illness are moderately heritable.
This has two implications

• Individuals actively shape their environments through heritable behaviour
• The relationship between environmental exposure and psychopathology may be confounded by genotype

There are three types of genotype-environment correlation

• Passive
  The association between the genotype a child inherits from his or her parents and the environment in which the child is raised.
• Evocative (or reactive)
  The association between an individual’s genetically influenced behaviour and others’ reactions to that behaviour.
• Active (or selective)
  The association between an individual’s genetic propensities and the environmental niches that individual selects.

These forms of genotype-environment correlation differ from gene-environment interaction (GxE), which refers to genetic differences in sensitivity to particular environmental effects.

• Genotype-environment correlations explain why individuals have a genetic propensity to engage in sensation-seeking behaviours affiliate with drug-abusing peers.
• GxE explains why heavy drug use is most likely to lead to psychosis only among individuals with a particular genotype.

Evidence from the quantitative genetic literature

Twin and adoption studies demonstrated that putative environmental measures are heritable.
These include many environments that are associated with psychiatric illness, including:

• Marital quality
• Social support
• Parental discipline and warmth
• Family environment
• Peer relationships
• Desirable and undesirable life events
  • Divorce
  • Exposure to trauma

The weighted heritability of these environments ranges from 6 to 39%, with most ranging from 15 to 35%.

When a study involves child twins reporting their experiences, genetic influences on the putative environment reflect the extent to which the child’s genetic propensities elicit or evoke that experience.
When studies involve samples of adult twins reporting their experiences, genetic influences on the putative environment reflect the extent to which the adult’s genetic propensities modify or create that experience.

Environments are heritable because genotype influences the behaviours that evoke, select, and modify features of the environment.
Environments less amenable to behavioural modification tend to be less heritable.

Evidence from the molecular genetic literature

Molecular studies measure genotype directly.
It may be possible to identify specific genotypes that correlate with environmental variables.
Significant genotype-environment correlations are relatively uncommon.

Implications for disease prevention

The existence of genotype-environment correlation raises two possibilities with implications for the prevention and treatment of psychiatric illness

• The relationship between psychological risk factors and psychiatric outcomes is not causal but confounded by genotype
  • Modifying the putative risk environment will have little effect on psychiatric illness
• The relationship between psychological risk factors and psychiatric outcomes is causal
  Modifying exposure to the risk environment should reduce levels of psychiatric illness

In some cases, a causal link between psychosocial risk factors and psychiatric outcomes can be established, even if exposure to the risk environment is under genetic influence.
The origins of a risk factor and the mechanism by which its effects are exerted are separable and may be distinct.
There are often reciprocal relationships between psychosocial risk factors and psychiatric outcomes.

Form many disorders, successful treatment will need to be targeted at multiple levels.